1. Field of the Invention
The present invention relates to electrodes for solid oxide electrochemical cells and, more specifically, to improved materials for fabricating cermet electrodes on solid oxide fuel cells by electrochemical vapor deposition. In this case, electrochemical cells include fuel cells, electrolyzers and sensors that operate on the basis of electromotive force measurement and/or current measurement and which comprise a solid oxide electrolyte and attached electrodes. Solid oxide fuel cells are one typical field of application f this invention. Although this invention was developed specifically for the fabrication of electrodes on fuel cells, it may also be used to fabricate electrodes on a variety of other electrochemical devices.
2. Description of the Prior Art
Solid oxide fuel cells are high temperature electrochemical devices fabricated primarily from oxide ceramics. Typically, they contain an oxygen ion conducting solid electrolyte, such as stabilized zirconia. The electrolyte is usually a this dense film which separates two porous electrodes--an anode and a cathode. The cathode, which is maintained in an oxidizing atmosphere, is usually an oxide doped for high electrical conductivity, such as strontium doped lanthanum manganite. The anode, on the other hand, is maintained in a reducing atmosphere and is usually a cermet such as nickel-zirconia. Finally, an interconnection is usually employed which is a dense, electronically conducting oxide material which is stable in both reducing and oxidizing environments, such as doped lanthanum chromite. The interconnection is deposited on a cell as a thin gas-tight layer in such a manner that it permits the anodes and cathodes of adjacent cells to be electrically connected in series. The gas-tightness of the interconnection, in combination with that of the electrolyte, ensures that the entire cell is gas-tight, preventing mixing of the anode and cathode atmospheres.
Solid oxide cells can be operated in either an electrolysis mode or in a fuel cell mode. In an electrolysis mode, DC electrical power and steam or carbon dioxide or mixtures thereof are supplied to the cell which then decomposes the gas to form hydrogen or carbon monoxide or mixtures thereof, as well as oxygen. In the fuel cell mode, the cell operates by electrochemically oxidizing a gaseous fuel such as hydrogen, carbon monoxide, methane or other fuels to produce electricity and heat.
The use of nickel-zirconia cermet anodes for solid oxide electrolyte fuel cells is well known in the art, and taught, for example, by A. O. Isenberg in U.S. Pat. No. 4,490,444. The anode must be compatible in chemical, electrical, and physical-mechanical characteristics such as thermal expansion, to the solid oxide electrolyte to which it is attached. A. O. Isenberg, in U.S. Pat. No. 4,597,170 solved bonding and thermal expansion problems between the anode and solid oxide electrolyte, by use of a skeletal embedding growth, of for example, primarily ionically conducting zirconia doped with minor amounts of yttria. The skeletal growth extends from the electrolyte/anode interface into a porous nickel layer, with the composite structure comprising the porous cermet anode.
Anchoring of the porous nickel anode to the solid oxide electrolyte was accomplished by a modified electrochemical vapor deposition (EVD) process. While this process provided well-bonded anodes, having a good mechanical strength and thermal expansion compatibility, gas diffusion overvoltages were observed during operation, lowering overall cell performance.
In order to reduce gas diffusion overvoltages, A. O. Isenberg et al., in U.S. Pat. No. 4,582,766, taught oxidizing the nickel in the cermet electrode to form a metal oxide layer between the metal, and the electrolyte, the embedding skeletal member. Subsequent reduction of the metal oxide layer forms a porous metal layer between the metal, and the electrolyte and skeletal member allowing greater electrochemical activity.
U.S Pat. No. 4,894,297 (Singh et al.), taught impregnating the cermet fuel electrode with chemicals that form metal oxides upon heating, for example Mg, Ca+Al, Sr+Al, Zr, Y and Ce salts, in order to prevent carbon deposition from hydrocarbon fuel. Further improvements were made in U.S. Pat. No. 4,767,518 (Maskalick) which taught discrete deposits of praseodymium oxide, dysprosium oxide and terbium oxide, present in the range of from 0.1 wt % to 5 wt % in the cermet electrode, to reduce cell over-potential and increase cell efficiency.
While it has been established that fuel cell anodes fabricated by the EVD process provide fuel cells with acceptable performance, one aspect of the EVD process can be problematical. Specifically, the EVD process is performed by applying a nickel powder layer on the electrolyte of a cell, using an aqueous powder slurry containing an organic binder. After drying, the cell is then heated to the EVD operating temperature. During heating, the nickel powder layer sinters, thereby consolidating the nickel powder into a porous body. Some degree of sintering is required to develop adequate electrode mechanical strength and electrical conductivity. Once the operating temperature is reached, EVD is performed and zirconia is deposited throughout the electrode. This zirconia then effectively prevents any further sintering of the electrode.
Occasionally, problems with electrode peeling and/or splitting during heating to the EVD operation temperature have been encountered. Electrode peeling and splitting are both the result of stresses produced in the electrode during sintering. Such stresses occur because the sintering of the electrode is constrained by the fuel cell on which it is applied. If it were free-standing, the electrode would shrink continuously as it sintered. Because the shrinkage of the electrode is inhibited by being applied to an essentially non-sintering substrate, surface tractions develop at the interface between the electrode and the electrolyte. These tractions are balanced by stresses induced in the electrode layer.
The stresses in the electrode give rise to a peeling force at electrode edges and tensile stresses within the bulk of the electrode layer. The latter stresses can exceed electrode strength and cause splitting of the electrode. While a certain degree of sintering is essential to develop required electrode mechanical and electrical properties, the occurrence of electrode peeling and splitting indicates that an excessive amount of electrode sintering can occur. What is needed is a method for controlling the degree of sintering of the electrode during heating to the EVD operation temperature. A main objective of this invention is to provide such a method.